Modern sealants are comprised of pigmented or unpigmented synthetic elastomeric polymers, which, in the non-cured state, constitute pourable or easily extrudable putty-like mastics. When cured, they are transformed into elastomeric materials which seal these structural elements and are able to contract or expand with the motion of the connected elements. They also form a tight barrier against moisture, gases and chemicals.
Such sealants include one-component and two-component compositions whose curing may proceed according to several mechanisms. One type of sealant, urethane sealants, include one-component compositions containing only isocyanate terminated prepolymers that cure by reaction of mixtures and isocyanate to form urea linkages. Two-component urethane sealants are prepared from isocyanate-terminated prepolymers and compounds containing active hydrogens both of which are generally sufficiently fluid to be blended at room temperature. These two component sealants cure by the formation of urethane linkages. Urethane polymers are good candidates for sealant applications because they possess a number of desirable properties including outstanding abrasion resistance, oil resistance, low temperature flexibility, resistance to degradation, excellent elasticity and, consequently, a high recovery. Because of an almost unlimited diversity of raw materials, polyurethanes can be formulated with excellent physical properties including low modulus, with retention of good tear and tensile strength.
Recently sealants which expand when heated have become available. These foam sealants, because they are able to expand to effectively seal a joint, are particularly useful as sealants for irregularly shaped or poorly fitting joints. Flexible urethane foams are generally prepared by incorporating water into the urethane composition. The reaction of the water and the isocyanate generates carbon dioxide which acts as a blowing agent to generate the foam. Alternately, fluorocarbons such as trichlorofluoromethane or other volatile solvents have been incorporated into polyurethane compositions as blowing agents to form flexible foams.